Controlled power fade for battery powered devices

ABSTRACT

A method is provided for operating a power tool having a motor powered by a battery. The method includes: delivering power from the battery to the motor in accordance with an operator input; detecting a condition of the power tool indicating a shutdown of the power is imminent; and fading the power delivered from the battery to the motor, in response to the detected condition, through the use of a controller residing in the power tool.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Utility Application No.13/080,712 filed on Apr. 6, 2011, which claims the benefit of U.S.Provisional Application No. 61/321,699 filed on Apr. 7, 2010. Thedisclosure of the above application is incorporated herein by reference.

FIELD

The present disclosure relates to portable electronic devices powered bybatteries and, more particularly, to methods for controlling power fadeduring operation of such devices.

BACKGROUND

Cordless products or devices which use rechargeable batteries areprevalent in the marketplace. Rechargeable batteries may be used innumerous devices ranging from computers to power tools. Since thedevices use a plurality of battery cells, the battery cells are commonlypackaged in a battery pack. The battery pack may in turn be used topower the devices when coupled thereto. Once depleted, the battery packmay be recharged by a battery charger

Over the past few years, lithium-ion (Li-ion) batteries have begunreplacing nickel-cadmium (NiCd), nickel-metal-hydride (NiMH), andlead-acid batteries in such portable electronic devices. As compared toNiCd and NiMH batteries, Li-ion batteries are lighter but have a largercapacity per unit volume. For this reason, the Li-ion batteries aresuitable to devices that are preferably light and which are required toendure continuous use for a long time. Li-ion batteries, however, maydeteriorate rapidly when subjected to overcharging, over-discharging,overheating, or other adverse conditions. Therefore, these types ofdevices employ protective measures to prevent such adverse conditions.Upon detecting an adverse condition, the system can be designed toabruptly terminate discharge of current from the batteries and therebycease device operation.

Therefore, it is desirable to develop a protection scheme that prolongsoperation of the device before current discharge is terminated and/orwarn the operator that the device is approaching a condition whichrequires terminating current discharge. This section provides backgroundinformation related to the present disclosure which is not necessarilyprior art.

SUMMARY

A method is provided for operating a power tool having a motor poweredby a battery. The method includes: delivering power from the battery tothe motor in accordance with an operator input; detecting a condition ofthe power tool indicating a shutdown of the power is imminent; andfading the power delivered from the battery to the motor, in response tothe detected condition, through the use of a controller residing in thepower tool.

A power tool system is presented that fades the power delivered by abattery upon detecting a condition of the power tool indicating ashutdown of the power is imminent. The power tool includes: a toolassembly having a motor; a battery pack that selectively couples to thetool assembly and operates to provide power to the motor; and adischarge control module that monitors a parameter indicative of tooloperation while power is delivered from the battery pack to the motorand fades the power delivered from the battery to the motor by an amountthat is computed as a function of a value of the parameter.

In another aspect of this disclosure, a method is provided for operatinga power tool powered by a battery having a plurality of battery cells.The method includes: measuring voltage of the battery while current isbeing drawn from the battery; interrupting current momentarily tomeasure voltage of the battery in an unloaded condition; measuringvoltage of the battery in an unloaded condition; comparing the unloadedvoltage measure to a voltage cutoff threshold; and resuming current drawfrom the battery when the unloaded voltage measure exceeds the voltagecutoff threshold. When the unloaded voltage measure is less than thevoltage cutoff threshold, current discharge from the battery isterminated.

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

FIG. 1 is a schematic illustrating an exemplary embodiment of a powertool;

FIG. 2A is a block diagram illustrating inputs and outputs to adischarge control module;

FIG. 2B is a flow diagram illustrating an exemplary embodiment of acontrol loop;

FIG. 3 is a flow diagram illustrating an exemplary embodiment of amethod to determine a PWM duty cycle based on a measured parameter;

FIG. 4 is a flow diagram illustrating an exemplary embodiment of amethod to determine a PWM duty cycle based on a battery pack temperaturereading;

FIG. 5 is a graph illustrating the relationship between the PWM dutycycle and the temperature of the battery pack;

FIG. 6 is a flow diagram illustrating an exemplary embodiment of amethod to determine a PWM duty cycle based on a FET temperature reading;

FIG. 7 is a graph illustrating the relationship between the PWM dutycycle and the temperature of the FET switch;

FIG. 8 is a flow diagram illustrating an exemplary embodiment of amethod to determine a PWM duty cycle based on a battery pack voltagetemperature reading;

FIG. 9 is a graph illustrating the relationship between the PWM dutycycle and the capacity of the battery pack;

FIG. 10 is a flow diagram illustrating an exemplary method formonitoring a battery pack at tool startup;

FIG. 11 is a graph illustrating an exemplary relationship betweentemperature and blanking time;

FIG. 12 is a graph depicting typical discharge curves for a lithium ioncell being discharged at different current loads; and

FIG. 13 is a flowchart illustrating an exemplary embodiment of a voltagecutoff scheme for use in a power tool application;

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure. Further areas ofapplicability will become apparent from the description provided herein.The description and specific examples in this summary are intended forpurposes of illustration only and are not intended to limit the scope ofthe present disclosure. Corresponding reference numerals indicatecorresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

FIG. 1 depicts an exemplary configuration of a power tool assemblycomprised of a power tool 12 and a battery pack 14. The power toolassembly may be comprised of a tool instrument (not shown) which isdriven by a motor 22. The motor 22 is controlled by a discharge controlmodule 20. The discharge control module 20 monitors various conditionsof the power tool and the battery pack and controls the power output tothe motor accordingly. The exemplary configuration is merely provided asa context for describing the concepts presented below. It is understoodthat the configuration may represent only a portion of the internalcircuitry and thus may include additional functionality or componentssuch as protection circuits which are not shown herein for reasons forclarity.

In the exemplary embodiment, the battery pack 14 is removably coupledvia cell tabs or terminals (not shown) to the power tool 12. The batterypack 14 includes three battery cells, 32A, 32B and 32C although more orless cells are contemplated. For purposes of describing the exemplaryembodiments, the battery pack 14 employs cells having lithium-ion cellchemistry. It is understood that the battery pack 14 may be composed ofcells of another lithium-based chemistry, such as lithium metal orlithium polymer, or another chemistry such as nickel cadmium (NiCd),nickel metal hydride ride (NiMH) and lead-acid, for example.Additionally, other means of coupling the pack 14 to the tool 12 areenvisioned or the pack 14 may be integrated into the power tool 12.

The motor 22 is controlled by an operator via an on/off variable speedtrigger switch 24, which when engaged by the user, closes a switchcoupled to the variable speed trigger switch 24. The variable speedtrigger switch 24 may be pressed in by the operator using variouspressures and the rotational speed of the motor 22 corresponds to thetotal distance traveled by the variable speed trigger switch 24. Forexample, when the trigger switch 24 is pressed all the way in, absent afade condition (discussed below), the rotational speed of the motor 22will be at full capacity. As the amount of total distance traveled bythe trigger switch 24 decreases, the rotational speed of the motor 22will decrease as well. As will be discussed, this can be achieved bypulse width modulating the power signal to the motor 22.

In power tools where a reverse operation is preferred, e.g. ascrewdriver or drill, the power tool 12 further includes a 2 pointdouble-pole-double-throw (DPDT) switch 28, whereby the polarity of thecircuit is reversed by throwing the DPDT switch 28. As may beappreciated, the power tool operator may manually push a button orswitch on the exterior of the power tool to cause the motor 22 tooperate in reverse. When the operator manually pushes the reverse buttonor switch, the DPDT switch 28 is thrown and the motor 22 will rotate inthe opposite direction.

Furthermore, the power tool assembly may include an LED (Not shown). TheLED may be lit when the power tool is in operation. The LED may becoupled to the discharge control module 20, so that the LED is providedwith power when the variable trigger switch 24 is engaged by a user.

In the exemplary embodiment, the discharge control module 20 is locatedin the power tool 12. This configuration, although not required,decreases the cost of a battery pack as well as allows for differenttools to use the same type of battery pack. The discharge control module20 controls the power being delivered to the motor 22, which drives thepower tool instrument (not shown). When the operator engages the triggerswitch 24, discharge control module 20 sets the duty cycle of a controlsignal is set in accordance with position of the trigger switch 24. Morespecifically, the discharge control module 20 provides a pulse widthmodulated control signal to FET 25 which in turn closes the circuit sopower is delivered to the motor 22. For explanatory purposes, the switchis hereinafter referred to as a trigger switch 24, but it is understoodthat the switch 24 may be other types of switches. When a cutoffcondition exists, e.g. the battery temperature increases above atemperature cutoff threshold, the discharge control module 20 terminatesthe control signal to the FET 25, thereby opening the circuit anddepriving the motor 22 of power. While the following description isprovided with reference to a discharge control module residing in thetool, it is understood that broader aspects of this disclosure can beimplemented by a control module residing in the battery pack.

When an operator disengages the trigger switch 24, the discharge controlmodule 20 can shut off the motor 22 by, for example, closing a brakeswitch 23 thereby shorting the motor. In some embodiments, the brakeswitch 23 is mechanically coupled to the variable speed trigger switch24 so that once the switch is opened, the brake switch is closed, whichshorts the motor 22.

The discharge control module 20 also monitors various conditions of thepower tool 12 and the battery pack 14 to determine if a conditionrequiring a tool shutdown is imminent. Upon detecting such condition,the discharge control module may initiate a fading of the powerdelivered from the battery to the motor which may be referred to hereinas a power fade. A power fade is when the total power output to themotor is slowly decreased as the condition being monitored approaches acutoff condition. When a power fade is initiated, the power beingdelivered to the motor 22 is gradually decreased, so as to eitherprolong the use of the power tool until the cutoff condition is reachedor to avoid the cutoff condition altogether. Furthermore, as the tooloperator observes the power slowly decreasing, the tool operator will beaware that a cutoff condition is approaching. In operation, thedischarge control module 20 can execute a main control loop that callsvarious subroutines to monitor various conditions of the power tool andbattery pack. For purposes of explanation, the condition where a toolshutdown is approaching but not yet reached, such that a power fade isinitiated, is hereinafter referred to as a fade condition.

For example, the discharge control module 20 can monitor the voltage ofthe battery pack. As shown in FIG. 1, the voltage of the battery pack 14can be measured by discharge control module 20 at, for example, node 35.The voltage is read by the discharge control module 20 so that thevoltage of the battery cells can be monitored. If the voltage fallsbelow a fade voltage threshold, e.g. 10.5V, the discharge control module20 initiates a power fade, the details of which are described below.Once the battery pack reaches a cutoff voltage, e.g. 10V, the dischargecontrol module 20 cuts power to the motor 22 by turning the FET 25 off.Further a split cell voltage may be monitored at nodes 34 or 36.Extending from nodes 34 and 36 are cell taps extending from each node. Acell tap is a wire or other connection that couples the nodes 34 and 36to the discharge control module 20.

To monitor the battery pack 14 temperature, a temperature sensor 30 isused. One example of a temperature sensor is a thermistor, which is acost effective yet dependable means of monitoring the temperature in acircuit. It is envisioned, however, that other types of temperaturesensors may also be used, e.g. thermometer or thermocouple. Thetemperature sensor 30 provides a reading of the battery pack temperatureto the discharge control module 20. As will be described below, when thetemperature of the battery pack 14 reaches a fade temperature threshold,which is less than the cutoff temperature threshold, the dischargecontrol module 20 initiates a power fade. A second temperature sensor 26is used to measure the temperature of the power tool 12. Similar to thefirst temperature sensor, a thermistor, thermometer, or a thermocouplemay be used to measure the temperature.

While the foregoing describes some embodiments of the power tool 12, aswell as the battery pack 14 coupled thereto, it is envisioned that otherconfigurations can be implemented in the design of the power tool 12 andbattery pack 14. Further, it is appreciated that the aspects of theinvention discussed below may be applied to a wide variety of platformsand are not solely limited to the configuration described above.

One means for delivering power to the motor is by performing pulse widthmodulation (PWM) on the power signal or control signal sent to the FET25. Pulse width modulation is a means of delivering varying amounts ofpower to the motor. The duty cycle of the control signal dictates theaverage value of the voltage delivered to the motor. Thus, a duty cycleof 80% corresponds to an 80% voltage output, which is obtained by havingthe voltage ON for 80% of the cycle and OFF for 20% of the cycle. Thiscan be achieved by having the FET 25 closed 80% of the time and open theremaining 20% of the time. Thus, by controlling the value of the PWMduty cycle various voltage outputs can be achieved.

As discussed above, the total distance traveled by the variable speedtrigger switch 24 can affect the rotational speed of the motor 22. Therotational speed of the motor 22 is controlled by setting the PWM dutycycle equal to a value based on the desired rotational speed of themotor 22. For instance if the operator presses the trigger the maximumdistance, the PWM duty cycle will initially be set to 100%. As thepressure applied to the trigger 24 decreases, thereby indicating thatthe operator wishes to slow the motor 22, the PWM duty cycle isdecreased as well.

Moreover, if a fade condition is realized, the PWM duty cycle can bedecreased to prolong the use of the motor before the cutoff condition isreached. As will be described below, the discharge control module 20will monitor various components of the tool, and if any of thecomponents are approaching a cutoff condition, i.e. are in a fadecondition, the discharge control module 20 calculate a PWM duty cyclebased on a value corresponding to the component in a fade conditionstate. It is appreciated that the PWM duty cycle that controls the motor22 is set equal to whichever is less of the PWM duty cycle correspondingto the trigger switch 24 or the PWM duty cycle corresponding to thecomponent in a fade condition state.

In most situations, a fade condition will not be realized. In theseinstances, the PWM duty cycle can be set to 100% or any other valuecorresponding to the distance traveled by the trigger switch 24. Inother situations, a cutoff condition will be realized. In theseinstances, the PWM duty cycle should be set to 0%. In the situationswhere a fade condition has not yet reached a cutoff condition, the PWMduty cycle can be set between a maximum PWM duty cycle, e.g., 100%, anda minimum PWM duty cycle, e.g., 20% or 0%. Further, it is envisionedthat as the value of a monitored parameter approaches a cutoffcondition, the duty cycle may be slowly decreased, thereby resulting ina power fade of the power output. As mentioned above, if the userreleases the trigger switch 24 the position of the trigger switch 24 mayresult in a PWM duty cycle which is less than a duty cycle correspondingto a fade condition.

FIG. 2A depicts exemplary inputs and outputs to the discharge controlmodule 20. The discharge control module 20 receives signalsrepresentative of various measured parameters from various sensors ornodes. In this example, the discharge control module 20 receives abattery pack temperature signal 50 from a battery pack temperaturesensor 30, a stack voltage signal 52 from a node associated with thebattery pack, a tool assembly temperature signal 54 from a tool assemblytemperature sensor and a FET drain voltage 56 from the FET. Thesemeasured parameters are exemplary and are in no way intended to belimiting. It is appreciated that other measured parameters may bemonitored by the discharge control module 20, such as motor speed, motortorque, tool rotation etc.

The discharge control module 20, when the tool is turned on, may executea control loop to monitor the various measured parameters and based onthe measured parameters will determine a PWM duty cycle for the powersignal 56. The discharge control module 20 determines whether any of themeasured parameters are below any of the fade condition thresholds forthe monitored conditions, e.g. whether the battery pack temperature istoo high. If so a fade PWM duty cycle can be calculated based on themonitored sensor value. If two or more measured parameters necessitate apower fade, then the lowest calculated fade PWM duty cycle can be set tothe actual PWM duty cycle.

FIG. 2B demonstrates the main control loop which starts executing atstep 60, when the power tool is turned on by the user. The control loopwill continue to execute until the power tool is shut down or until thedischarge control module 20 is operated in a low power consumption mode,as is described below. In the latter scenario, the discharge controlmodule 20 may be returned to a full power consumption mode using awatchdog timer. At steps 62-68, various methods or applications areinitiated by the control loop to monitor the various measured parametersand control the PWM duty cycle of the discharge control module 20 basedon the measured parameters. The foregoing is provided for an example ofa main control loop and the number of methods called by the control loopis purely provided for example only. Furthermore, while FIG. 2B depictsthe methods executing in series, it is envisioned that the methods mayalso execute in parallel or concurrently.

FIG. 3 depicts an example method that may be executed by the dischargecontrol module 20. The discharge control module 20 receives signalsrepresenting values of various measured parameters and determines if aparticular parameter value is below a corresponding fade thresholdvalue. If the particular parameter value is below the corresponding fadethreshold value, i.e. the power tool is in a fade condition, then a PWMduty cycle is calculated based on the measured parameter. The calculatedPWM duty cycle is compared to the actual PWM duty cycle to determine ifthe actual PWM duty cycle needs to be set equal to the PWM duty cycle.

At step 100, the control loop will call the subroutine to determine if apower fade should be performed. The discharge control module 20 receivessignals representing values of various measured parameters from any ofthe various sensors or nodes in the power tool or the battery pack atstep 110. For example, the discharge control module 20 may receivesignals from a temperature sensor monitoring one of the battery pack 14temperatures, the FET 25 temperature, and a voltage reading from a nodebetween two of the battery cells. It is envisioned that other conditionsof the power tool or battery pack may be monitored as well.

At step 112, the measured parameter is compared with a fade threshold.For example, a temperature reading from the battery pack may be comparedto a battery pack fade temperature threshold. In this example, if thetemperature reading exceeds the battery pack fade temperature threshold,then the fade condition is met, and the method proceeds to step 114. Ifthe fade condition is not met, the method steps back to the controlloop.

Next, if the fade condition is met, the fade PWM duty cycle iscalculated at step 114. Depending on the type of fade condition, thecalculation may vary. Examples relating to calculated PWM duty cyclesfor particular fade conditions are provided below. One general exampleis to calculate a linear equation representing the PWM duty cycle as afunction of the monitored condition. Knowing the maximum PWM duty cycle,e.g. 100%, the minimum PWM duty cycle, e.g. 20% or 0%, the fadeconditions corresponding to maximum and minimum PWM duty cycles, and themeasured parameter, the fade PWM duty cycle can be calculated asfollows:

$\begin{matrix}{{FadePWM} = {{MaxPWM} - {\frac{{MaxPWM} - {MinPWM}}{{FCEnd} - {FCStart}}\left( {{MC} - {FCStart}} \right)}}} & (1)\end{matrix}$wherein FCStart is the start of the fade condition, i.e. at what valueof the measured parameter is the power fade initiated, FCEnd is the endof the fade condition, and MC is the measured parameter value. It isenvisioned that various models for calculating a fade PWM duty cycle maybe incorporated. While this disclosure discusses calculatingsubstantially linearly decaying PWM duty cycles, a step model, apolynomial model or an exponential decay model may also be implemented.Once the PWM duty cycle is calculated, the method proceeds to step 116.

The actual PWM duty cycle, i.e. the PWM duty cycle that the power toolis currently operating at, is compared to the calculated fade PWM dutycycle at step 116. If the actual PWM cycle is greater than thecalculated fade PWM duty cycle, then the actual PWM duty cycle is set tothe calculated PWM duty cycle at step 118. If the actual PWM cycle isnot greater than the calculated fade PWM duty cycle, then the subroutinereturns to the control loop.

The decision not to update the actual PWM duty cycle to the calculatedfade PWM duty cycle when the fade PWM duty cycle is less than the PWMduty cycle determined by trigger position is based on the assumptionthat the PWM duty cycle may already be set below the calculated fade PWMduty cycle because another measured parameter may be closer to thecutoff condition than the sensor measured parameter analyzed in thesteps described above. For example, if the battery cell is almost whollydepleted and the actual PWM duty cycle was set to reflect thiscondition, then a battery pack temperature that is barely past the fadecondition threshold should not increase the duty cycle. In someembodiments, however, that actual PWM duty cycle could be set to thecalculated fade PWM duty cycle, regardless of the comparison of the twovalues.

In some embodiments, the method may be implemented so that when the PWMduty cycle is calculated, the PWM duty cycle is calculated to 0% when acutoff condition is met. However, in other embodiments, the measuredparameters are compared with predetermined cutoff conditions todetermine if a cutoff condition has been realized. If a cutoff conditionis realized, then the method may set the PWM duty cycle to 0% and themethod ends.

As can be appreciated this method may execute continuously while thepower tool is in use. Alternatively, the method may execute inpredetermined time intervals. Also, it is envisioned that othervariations of the method may be performed. For example, at step 112, ifno fade conditions exist, then the actual PWM duty cycle may be set to100%. This variation may be beneficial if a fade condition waspreviously realized, thereby initiating a power fade, but the fadecondition was resolved prior to recalculating the PWM duty cycle.

The method described generally above may be used to monitor variousconditions of the power tool and to protect the tool or battery packfrom damage. In previous tools, the practice was to allow the variousconditions to worsen while the tool operated at a 100% power outputuntil the cutoff condition occurred, whereupon the tool was shut downand left inoperable until the cutoff condition was remedied. Using thegeneral framework described above, however, may result in an increasedoperation time of the power tool as well as a prolonged life of the tooland/or the battery pack.

In some embodiments, the battery pack temperature may be monitored todetermine if the battery pack is in a fade condition. If the batterypack temperature has reached or has exceeded a fade condition threshold,a PWM duty cycle corresponding to the battery pack temperature iscalculated.

FIG. 4 illustrates a flow chart of a method that may be executed by thedischarge control module 20 to monitor the battery pack temperature. Atstep 200, the control loop calls the subroutine to monitor the batterypack temperature. At step 210 the temperature of the battery pack ismonitored. As previously mentioned, in some embodiments the temperaturesensor may be a thermistor. In these embodiments, the discharge controlmodule 20 measures a voltage at a pack thermistor input. The voltagemeasured at the pack thermistor input is a fraction of the battery packvoltage. From previously collected test data, the resistance of thethermistor at specific temperatures is known. Thus, by monitoring thevoltage at the thermistor input and the voltage at the battery pack, aratio of the battery pack voltage and the thermistor input voltage canbe calculated. It is appreciated that the ratio corresponds to theresistance of the thermistor at a particular temperature. Thus, thecalculated ratio may be used to query a look up table, whereby atemperature corresponding to the ratio is returned. It is appreciatedthat the look up table may be stored in non-volatile memory associatedwith the discharge control module 20.

At step 212 the method compares the determined battery pack temperaturewith the battery pack fade temperature threshold by comparing thevoltage reading at the thermistor input of the discharge control module20 with a voltage corresponding to the battery pack fade temperaturethreshold given the current voltage of the battery pack. It isunderstood that in some embodiments, the calculated ratio, i.e. theratio of the pack voltage to the thermistor input voltage, may becompared to a predetermined ratio corresponding to the battery pack fadetemperature. If the pack temperature is above the threshold, then themethod proceeds to step 214. If not, the method steps back to step 210.

At step 214 the fade PWM duty cycle is calculated. As discussed, the PWMduty cycle may be calculated using equation 1. In particular, the fadePWM duty cycle may be calculated using the following:

$\begin{matrix}{{FadePWM} = {{MaxPWM} - {\frac{{MaxPWM} - {MinPWM}}{V_{{TI}\_{End}} - V_{{TI}\_{Start}}}\left( {V_{{TI}\_{Measured}} - V_{{TI}\_{Start}}} \right)}}} & (2)\end{matrix}$where V_(TI) _(_) _(Start) is the voltage reading corresponding to thebeginning of a fade condition, V_(TI) _(_) _(END) is the voltage readingcorresponding to the end of a fade condition, and V_(TI) _(_)_(Measured) is the actual measured voltage at the thermistor input.Furthermore, if should be appreciated that if the method is executingthis step, then V_(TI) _(_) _(Start)≦V_(TI) _(_) _(Measured)≦V_(TI) _(_)_(END). Also, as previously mentioned, MaxPWM is the maximum PWM dutycycle, e.g. 100%, and MinPWM is the minimum PWM duty cycle, e.g. 20%. Asdiscussed, the voltage at the thermistor input is dependent not only ontemperature but also the remaining voltage of the battery cell. Thus,the values of V_(TI) _(_) _(Start) and V_(TI) _(_) _(END) will varybased on the battery cell voltage. The values may be retrieved from thepreviously collected data, which may be stored in the memory associatedwith discharge control module 20. It is appreciated that a look up tablecan be stored in the memory linking different battery pack voltagevalues to corresponding V_(TI) _(_) _(Start) and V_(TI) _(_) _(END)values. It is further appreciated, that actual measured temperatures ofthe battery pack can be used instead of the measured voltage values. Itis appreciated, that measured temperatures of pack can be used insteadof the measured voltage value. In this scenario, start temperature valueand an end temperature value would be used in place of V_(TI) _(_)_(Start) and V_(TI) _(_) _(END).

In an example embodiment, the fade condition may begin when the batterypack temperature is at 60° C. and end when the temperature is at 65° C.In this example, the maximum PWM duty cycle is 100% and the minimum PWMduty cycle is 20%. Thus, using equation (2), if the current temperatureof the battery pack is approximately 62° C., then the fade PWM dutycycle should be at approximately 68%. FIG. 5 depicts a graphdemonstrating an exemplary relationship between battery pack temperatureand the fade PWM duty cycle.

Once the fade PWM duty cycle is calculated, at step 216, the fade PWMduty cycle is compared against the actual PWM duty cycle, i.e. the dutycycle at which the tool is currently operating at. If the fade PWM dutycycle is less than the actual PWM duty cycle, then the actual PWM dutycycle is set to the fade PWM duty cycle at step 218. This is donebecause the temperature of the battery pack is increasing andapproaching the cutoff temperature. If the fade PWM duty cycle isgreater than the actual PWM duty cycle, then the method steps back to210 and the actual PWM duty cycle is left unchanged. This may occur whenthe temperature of the battery pack begins decreasing as a result of thefade. It is envisioned that in some embodiments, however, that a reversefade may be implemented, whereby the actual PWM cycle is always replacedby the calculated fade PWM duty cycle, such that as the temperaturedecreases, the PWM duty cycle increases.

It is envisioned that this method may run continuously or atpredetermined time intervals. Furthermore, in some embodiments, thedischarge control module 20 may check whether the voltage reading at thethermistor input is below a cutoff condition. In these embodiments, ifthe voltage reading is below the cutoff threshold, then the power may becutoff until the battery pack temperature decreases below the cutoffthreshold. Further, it is appreciated that the thresholds provided inthe example above are provided for example only and are not intended tobe limiting. Additionally, the order of the steps is not mandatory andit is appreciated that the steps may be executed in different orders.Furthermore, it is envisioned that alternative steps may be taken andnot every step is essential.

In another aspect of the disclosure the switch assembly temperature maybe monitored. It is appreciated that when the power tool is used forprolonged periods the temperature of the FET 25, or other components inthe power tool circuit may increase to a point which may be damaging tothe those components. Thus, the temperature of the power tool may bemonitored and the PWM duty cycle of the power signal may be faded oncethe temperature reaches a value necessitating a power fade.

FIG. 6 illustrates a flow chart of a method that may be executed by thedischarge control module 20 to monitor the switch assembly temperature.In some embodiments, the discharge control module 20 monitors thetemperature of the FET 25 shown in FIG. 1. The temperature sensor may bea thermistor 26 disposed adjacent to the FET 25, whereby the dischargecontrol module 20 measures a voltage at a second thermistor input. Asopposed to the battery pack temperature sensor, the thermistor isconnected to a constant voltage source. Thus, the voltages correspondingto the switch temperature fade threshold can be hard coded, as thevoltage values corresponding to a start of a power fade and the end of apower fade will not vary as the voltage of the battery pack varies. Withthe exception of this caveat, the steps of FIG. 6 substantiallycorrespond to the steps of FIG. 4.

At step 300 the control loop calls the temperature monitoringsubroutine. At step 310, the voltage at the second thermistor 26 inputis measured. At step 312, the measured voltage is compared to thevoltage corresponding to the switch temperature fade threshold. If thevoltage is less than the voltage corresponding to the switch temperaturefade threshold, then a PWM duty cycle is calculated based on the switchtemperature and the method steps to 314. If the voltage is greater thanthe voltage corresponding to the switch temperature fade threshold, thenthe switch temperature has not yet passed the fade threshold, and themethod steps back to step 300.

At step 314 the fade PWM duty cycle is calculated. The fade PWM dutycycle is calculated based on a gain value and an offset value. The gainvalue can be calculated as follows:

$\begin{matrix}{{Gain} = \frac{{MaxPWM} - {MinPWM}}{V_{{TFET}\_{Start}} - V_{{TFET}\_{End}}}} & (3)\end{matrix}$The offset value can be calculated as follows:Offset=Gain×V _(TFET) _(_) _(Start)−MaxPWM  (4)Using the gain and the offset, the fade PWM duty cycle can be calculatedas follows:FadePWM=Gain×V _(TFET) _(_) _(Measured)−Offset  (5)It is noted that the gain and the offset values can be based on fixedvalues so that in some embodiments, the gain and the offset values canbe hard coded in the memory associated with the discharge control module20. In these embodiments, the fade PWM duty cycle can be calculatedbased on the received voltage measurement. In other embodiments, thefade PWM can be calculated using the following formula:

$\begin{matrix}{{FadePWM} = {{MaxPWM} + {\frac{{MaxPWM} - {MinPWM}}{V_{{TFET}\_{Start}} - V_{{TFET}\_{End}}}\left( {V_{TFET\_ Measured} - V_{TFT\_ Start}} \right)}}} & (6)\end{matrix}$where V_(TFET) _(_) _(Start) is the voltage value corresponding to thebeginning of a fade condition, V_(TFET) _(_) _(END) is the voltage valuecorresponding to the end of a fade condition, and V_(TFET) _(_)_(Measured) is the actual measured voltage at the second thermistorinput. Also, as previously mentioned, MaxPWM is the maximum PWM dutycycle, e.g. 100%, and MinPWM is the minimum PWM duty cycle, e.g. 20%. Asdiscussed, the voltage at the second thermistor input is dependent ontemperature and not on the remaining voltage of the battery cell. Thus,the values of V_(TFET) _(_) _(Start) and V_(TFET) _(_) _(END) may behard coded based on expected voltages at the threshold temperatures. Itis appreciated, that measured temperatures of FET can be used instead ofthe measured voltage value. In this scenario, start temperature valueand an end temperature value would be used in place of V_(TFET) _(_)_(Start) and V_(TFET) _(_) _(END).

In an example embodiment, the fade condition may begin when the switchtemperature is at 95° C. and end when the temperature is at 100° C. Inthis example, the maximum PWM duty cycle is 100% and the minimum PWMduty cycle is 20%. Thus, using equations (3)-(5) or (6), if the currenttemperature of the battery pack is approximately 99° C., then the fadePWM duty cycle should be at approximately 36%. FIG. 7 depicts a graphdemonstrating the relationship between switch temperature and the fadePWM duty cycle.

Once the fade PWM duty cycle is calculated, it may be compared againstthe actual PWM duty cycle, i.e. the duty cycle at which the tool iscurrently operating at. If the fade PWM duty cycle is less than theactual PWM duty cycle, then the actual PWM duty cycle is set to the fadePWM duty cycle. This is done because the temperature of the switch isincreasing and approaching the cutoff temperature. If the fade PWM dutycycle is greater than the actual PWM duty cycle, then the method stepsback to 300 and the actual PWM duty cycle is left unchanged.

In some embodiments, the discharge control module 20 determines whetherthe voltage reading is below a cutoff condition. In these embodiments,if the voltage reading is below the cutoff threshold, then the power iscut until the switch temperature decreases below the cutoff threshold.It is envisioned that this method may run continuously or atpredetermined time intervals as determined by the control loop of thedischarge control module 20. Further, it is appreciated that thethresholds provided in the example above are provided for example onlyand are not intended to be limiting. Furthermore, the order of the stepsis not mandatory and it is appreciated that the steps may be executed indifferent orders. Furthermore, it is envisioned that alternative stepsmay be taken and not every step is essential.

In some of the embodiments using a thermistor as the temperature sensor30, the thermistor is located on the positive (+) side of the triggerswitch. In the prior art, battery pack thermistors are located on thenegative (−) side of the trigger switch 24. With the configuration shownin FIG. 1, however, placing the thermistor on the (−) side of thetrigger switch 24 results in a current leakage. As can be observed inFIG. 1, the trigger switch 24 is located on the (−) side of the powertool circuit. As a result, if the thermistor is placed on the (−) oftrigger switch 24, the current would flow from the battery pack, throughthe discharge control module 20 and through the thermistor, therebybypassing the variable speed trigger switch 24. By locating thethermistor on the (+) side of the trigger switch 24, the current leakagecan be avoided because the trigger switch 24 would need to be engagedfor current to flow.

To further prevent leakage, blocking diodes are used in someembodiments. Blocking diode 37 is placed between the thermistor 30 andthe discharge control module as shown in FIG. 1. Blocking diodes 38 and39 are also placed on the cell taps between nodes 36 and 34 respectivelyand the discharge control module 30. Diodes are devices that helpcontrol current flows within the circuitry, which help maintainisolation of power supplies within the circuitry. The blocking diodes,in the current configuration, help ensure that the current does not flowback to the cells from the discharge control module 20.

In another aspect of the disclosure, the discharge control module 20 maymonitor the voltage of the battery pack. As the battery pack isdischarged, the discharge control module 20 may initiate a power fadewhen the pack voltage reaches a predetermined voltage reading.

FIG. 8 illustrates a flow chart of a method that is executed by thedischarge control module 20 to monitor the battery pack voltage. At step400, the control loop calls the voltage monitoring subroutine. As shownin FIG. 1, the voltage of the battery pack is read from node 35,sometimes referred to as a tap. Thus at step 410, the voltage is readfrom node 35 to monitor the voltage of the battery pack. It is furtherappreciated that the voltage of the individual cells or combinations ofcells can also be determined. For example, the split stack voltage atnode 34 can be read to determine the voltage of battery cell 32C. Usingthe voltages from node 35 and 34 the voltages of battery cells 32A and32B can also be determined.

The voltage reading of the battery pack is compared to a voltage fadethreshold as step 412. If the voltage reading is below the voltage fadethreshold, then the method proceeds to step 414 where a PWM duty cycleis calculated based on the battery pack voltage. If the voltage readingis above the voltage fade threshold, then the battery still has enoughcapacity, and a power fade is not required. The voltage fade thresholdmay correspond to a battery capacity of 5% of the maximum chargeremaining, for example.

At step 414 the fade PWM duty cycle is calculated. The fade PWM dutycycle may be calculated based on a gain value and an offset value. Thegain value can be calculated as follows:

$\begin{matrix}{{Gain} = \frac{{MaxPWM} - {MinPWM}}{V_{{BC}\_{Start}} - V_{{BC}\_{End}}}} & (7)\end{matrix}$The offset value can be calculated as follows:Offset=Gain×V _(BC) _(_) _(Start)−MaxPWM  (8)Using the gain and the offset, the fade PWM duty cycle can be calculatedas follows:FadePWM=Gain×V _(BC) _(_) _(Measured)−Offset  (9)It is noted that the gain and the offset values can be based on fixedvalues so that in some embodiments, the gain and the offset values canbe hard coded in the memory associated with the discharge control module20. In these embodiments, the fade PWM duty cycle can be calculatedbased on the received voltage measurement. In other embodiments, thefade PWM can be calculated using the following formula:

$\begin{matrix}{{FadePWM} = {{MaxPWM} + {\frac{{MaxPWM} - {MinPWM}}{V_{{BC}\_{Start}} - V_{{BC}\_{End}}}\left( {V_{{BC}\;{\_{Measured}}} - V_{{BC}\;\_\;{Start}}} \right)}}} & (10)\end{matrix}$In the foregoing equations, V_(BC) _(_) _(Start) is the voltage value ofthe battery pack corresponding to the beginning of a fade condition,V_(BC) _(_) _(END) is the voltage value of the battery packcorresponding to the end of a fade condition, and V_(BC) _(_)_(Measured) is the actual measured voltage at node 34. Also, aspreviously mentioned, MaxPWM is the maximum PWM duty cycle, e.g. 100%,and MinPWM is the minimum PWM duty cycle, e.g. 20%.

In an example embodiment, the fade condition begins when the batterycell reaches 10.5V and ends when the battery cell is at 10V. In thisexample, the maximum PWM duty cycle is 100% and the minimum PWM dutycycle is 20%. Thus, using equations (7)-(9) or (10), if the currentvoltage of the battery pack is approximately 10.2V, then the fade PWMduty cycle should be at approximately 52%. Once the fade PWM duty cycleis calculated, the duty cycle is compared against the actual PWM dutycycle, i.e. the duty cycle that the tool is currently operating at. Ifthe fade PWM duty cycle is less than the actual PWM duty cycle, then theactual PWM duty cycle is set to the fade PWM duty cycle. The method thensteps back to 410.

FIG. 9 depicts a graph demonstrating the relationship between batterypack capacity and the fade PWM duty cycle, where the pack voltagecorrelates directly with the battery pack capacity. It is envisionedthat this method may run continuously or at predetermined time intervalsas determined by the control loop of the discharge control module.Further, it is appreciated that the thresholds provided in the exampleabove are provided for example only and are not intended to be limiting.Furthermore, the order of the steps is not mandatory and it isappreciated that the steps may be executed in different orders.Furthermore, it is envisioned that alternative steps may be taken andnot every step is essential. Additionally, the method may be applied tomonitor the voltages of the individual battery cells as well.

Although we have been discussing voltage fade as a method for indicatingthat a pre-determined cut-off condition is approaching via packtemperature, pack voltage, module temperature, tool rotation, motortorque etc., it is envisioned that other methods of indication to thetool user can be implemented as well such as an LED indication (i.e.,flashing the LED at varying rates based on monitors proximity to thecut-off limit). Additionally or alternatively, the motor can be made to‘pulse’ by varying the PWM on and off at specific times. These are justa few ideas of other ways to indicate to the user that a condition isoccurring.

In another aspect of the disclosure, the discharge control module 20 isconfigured to prevent erroneous readings at startup in cold conditions.As mentioned, when the battery pack reaches an under voltage condition,the discharge control module 20 may determine that a cutoff conditionhas been reached. In these instances, the power to motor 22 will be cut,thereby preventing the battery pack from entering into a deep dischargestate. One problem associated with rechargeable battery packs, however,is that an increased voltage drop under a load condition may be observedin cold temperatures, e.g. below 10° C. In these cold conditions, whenthe tool operator first turns the power on, a high in rush current willcause a relatively large voltage drop. Additionally, cold conditions maycause the impedance in the battery pack circuit to increase, furtherexacerbating the voltage drop.

To combat this problem, the discharge control module 20 is configured todisregard the voltage readings for a predetermined time period, e.g. 300ms, hereinafter referred to as a blanking time period. The blanking timeperiod is a period where the discharge control module 20 allows themotor to run without monitoring the measured parameters to determinewhether a shutdown condition is reached. During the blanking timeperiod, the discharge control module 20 will bias the FET 25, therebyallowing current to flow to the motor 22, without determining if a undervoltage condition exists. Once the blanking time period has passed, thedischarge control module 20 will resume the monitoring of the variousinput signals to determine if an under voltage condition exists, and maycutoff the motor if such a condition does exist.

One drawback with having a predetermined blanking time period, however,is that depending on the temperature the blanking time period may be tooshort or may be too long. If the blanking time period is too short, theblanking time period may lapse while the voltage reading remainsadversely affected by the cold condition. Consequently, the tool may beshutdown prematurely despite the battery pack actually having enoughvoltage to power the motor 22. On the other hand, if the blanking timeperiod is too long and the battery pack 14 is actually depleted, thenthe battery pack 14 may be allowed to discharge even though theremaining charge in the battery pack would necessitate a shutdown of thetool.

As a means of remedying the above-described situations, some embodimentsimplement a variable blanking time period that is dependent on factorsthat influence the readings of the voltage, such as temperature. Forexample, if the battery pack temperature is relatively low, then theblanking time period should be longer in duration than when the batterypack temperature is relatively high. Accordingly, the time period of theblanking time period can be dependent on the temperature of the batterypack.

FIG. 10 illustrates an exemplary method for monitoring a battery packupon startup. At step 1000 the power tool is turned on by the operator.At step 1002, the battery pack temperature is read from the temperaturesensor 30.

Once a battery pack temperature is determined, the discharge controlmodule 20 determines a blanking time period based on the temperature atstep 1004. The blanking time period can be determined a number of ways.For example, there may be a look up table stored in a memory associatedwith the discharge control module 20. In the look up table, varioustemperatures may have corresponding blanking time periods or temperaturethresholds for setting the blanking time period. For example, FIG. 11shows a graph of blanking time period as a function of temperature. Ascan be seen, once the temperature is less than 5° C., the blanking timeperiod is increased from 300 ms to 500 ms. In alternative embodiments,an equation can be used to determine the blanking period. It isenvisioned that a linear equation, a second or higher order equation, alogarithmic equation, exponential equation, or other type of equationmay be used to set the blanking time period.

Once the blanking time period is determined, the power tool operateswithout the discharge control module 20 monitoring the measuredparameters, including conditions of the battery pack, until the blankingtime period has passed. Once the blanking time period is finished, thedischarge control module 20 commences monitoring the measuredparameters, including the voltage of the battery pack at step 1008. Itis appreciated that the exact function to determine the blanking timeperiod may also depend on the battery cell chemistry and themanufacturer of the battery cells.

FIG. 12 illustrates typical discharge curves for a lithium ion cellbeing discharged at different current loads. Discharge of the cellshould occur just before the voltage drops off precipitously. Althoughan absolute voltage cutoff may be employed (e.g., 2.5 volts), cutoffwould occur prematurely at higher loads. For example, the curve labeled2600 represents current discharge at 20 amps. In this instance, currentwould be discharged for a very brief time before an absolute voltagecutoff of 2.5 volts was reached. Strategies for compensating the voltagecutoff values for current draw are known. These strategies includemeasuring current and altering the voltage cutoff threshold based on themeasured current. Techniques for measuring current require additionalcurrent measurement and conditioning circuits. Therefore, it would bedesirable to implement voltage cutoff schemes without having to directlymeasure current.

One technique for measuring current is to use the battery itself as ashunt. In this technique, current is interrupted periodically (e.g.,once every second). Battery voltage is measured while current isdischarged from the battery and while the current draw interrupted. Inan exemplary embodiment, the impedance of a battery cell is known to beon the order of 48 milliohms per cell. In this case, the current can becalculated as follows:Current=(battery voltage with current off−battery voltage with currenton)/48 milliohmsSince the cell impedance can change over time, the battery impedance canbe stored in a memory and updated periodically. For example, the batteryimpedance can be determined while the battery is being charged. Whilethe charger outputs a known current to the battery, the battery voltagecan be measured with the current on and with the current off. Thisvoltage variance can then be used to determine and adjust the storedbattery impedance value. Other techniques for determining and/orupdating the battery impedance are contemplated by this disclosure.

An alternative voltage cutoff scheme is further described with referenceto FIG. 13. During discharge of the battery, the current being drawnfrom the battery is periodically interrupted for a short period of time.This short period of time should be such that the user does not feel orotherwise detects the interrupt (e.g., 3-5 milliseconds). During thisinterrupt period, a voltage measure is taken of the battery. The voltagemeasure may be of a single cell, a portion of the cells or the entirestack. The voltage measure is then compared to a target cutoff voltage.The battery is not considered sufficiently discharged if the voltagemeasure is above the target cutoff voltage or rises above the targetcutoff voltage during the interrupt period. In this case, the interruptperiod ends and current draw from the battery resumes. On the otherhand, if the voltage measure is below the target cutoff, thendischarging of the battery is terminated. In this way, the restingvoltage of the battery is measured directly and current compensation canbe avoided.

In this exemplary embodiment, the voltage measure is taken periodicallyor when a measurement of the loaded voltage (i.e., when current is beingdrawn from the battery) drops below a loaded voltage threshold (e.g.,2.7 volts for a lithium ion cell). When the battery begins discharging,an interrupt timer is started at 2610 to achieve a periodic sampling(e.g., once per second). Other techniques for achieving a periodicsampling rate are also contemplated by this disclosure.

Next, a voltage measure is taken 2612 of the battery while the batteryis being discharged. The voltage measure may be of a single cell, aportion of the cells or the entire stack. In any case, the voltagemeasure is compared at 2614 to a loaded voltage threshold. Expiration ofthe interrupt timer is checked at 2616 if the voltage measure exceedsthe loaded voltage threshold. This cycle is repeated so long as thevoltage measure exceeds the loaded voltage threshold and the timer hasyet to expire.

Once the timer expires, the current draw from the battery is interruptedat 2632 and a second voltage measure is taken at 2634 during theinterrupt period. In the exemplary embodiment, there is an inherentdelay from when the current is interrupted until the time at which thesecond voltage measure is made by the microcontroller; this delay(preferably one the order of 2 ms) enables the voltage is return to itsresting value. In other embodiments, it may be necessary for the controlmodule to enforce a delay before taking the second voltage measure. Inany case, the second voltage measure is in turn compared to a targetcutoff voltage at 2636. The target cutoff voltage preferably is anunloaded voltage threshold (e.g., 2.7 volts for a lithium ion cell)although is may be the same as the loaded voltage threshold. If thesecond voltage measure is not below the target cutoff voltage, then thecurrent interrupt ends and discharging of the battery may resume. Inthis case, the interrupt timer is reset at 2630. If the second voltagemeasure is below the target cutoff, then discharging of the battery isterminated as indicated at 2640.

Likewise, if the voltage measure exceeds the loaded voltage threshold,then the current draw from the battery is interrupted at 2618 and asecond voltage measure is taken 2620 during the interrupt period. Thesecond voltage measure is again compared at 2622 to the target cutoffvoltage. If the second voltage measure is below the target cutoff, thendischarging of the battery is terminated as indicated at 2640.

On the other hand, if the second voltage measure is not below the targetcutoff, then the current interrupt ends and discharging of the batterymay resume. To avoid excessive voltage measurements while the loadedvoltage is below its threshold, a secondary timer (also referred toherein as a blank timer) is initiated at 2626. The battery voltage willnot be measured again until this secondary timer expires. Once thesecondary timer expires, control returns to step 2612 and the process isrepeated.

As used herein, the term module may refer to, be part of, or include anApplication Specific Integrated Circuit (ASIC), an electronic circuit, aprocessor (shared, dedicated, or group) and/or memory (shared,dedicated, or group) that execute one or more software or firmwareprograms, a combinational logic circuit, and/or other suitablecomponents that provide the described functionality.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention. Individual elements or features ofa particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the invention, and all such modificationsare intended to be included within the scope of the invention.

What is claimed is:
 1. A method for operating a power tool having atrigger switch and a motor powered by a battery pack, comprising:setting the duty cycle of a pulse width modulated control signal inaccordance with a position of the trigger switch; delivering power fromthe battery pack to the motor in accordance with the control signal;monitoring a parameter indicative of tool operation while power isdelivered from the battery pack to the motor using a controller residingin the power tool; detecting a condition of the parameter indicating ashutdown of the power is imminent; and fading the power delivered fromthe battery to the motor, in response to the detected condition, usingthe controller, wherein the fading step comprises: computing a value ofthe duty cycle as a function of a value of the parameter, and settingthe duty cycle of the control signal to the computed value of the dutycycle when the computed value of the duty cycle is less than the dutycycle set in accordance with position of the trigger switch, andretaining the duty cycle set in accordance with the position of thetrigger switch when the computed value of the duty cycle is more thanthe duty cycle set in accordance with the position of the triggerswitch.
 2. The method of claim 1 wherein fading of the power deliveredis by an amount that is computed as a function of a value of theparameter.
 3. The method of claim 1 wherein fading of the powerdelivered further comprises decreasing the power delivered as a functionof the value of the parameter, where the function is at least one oflinear, step, exponential or polynomial.
 4. The method of claim 1wherein the parameter is voltage of the battery, comprising: initiatingfading of the power delivered when the battery voltage falls below avoltage fade threshold and decreasing the power delivered linearly as afunction of the battery voltage until the battery voltage falls below avoltage cutoff threshold.
 5. The method of claim 1 wherein the parameteris temperature in the power tool, the method comprising: initiatingfading of the power delivered when the temperature exceeds a temperaturefade threshold and decreasing the power delivered linearly as a functionof the temperature until the temperature exceeds a temperature cutoffthreshold.
 6. The method of claim 1 wherein the parameter is temperaturein the battery pack, the method comprising: fading of the powerdelivered when the temperature exceeds a temperature fade threshold anddecreasing the power delivered linearly as a function of the temperatureuntil the temperature exceeds a temperature cutoff threshold.
 7. Themethod of claim 6, wherein the battery pack includes a positiveterminal, a negative terminal, a temperature sense terminal, batterycells coupled to the positive and negative terminals, and a temperaturesensor electrically coupled to the positive terminal.
 8. A power toolsystem comprising: a tool assembly having a motor; a battery pack thatselectively couples to the tool assembly and operates to provide powerto the motor; a trigger switch disposed on the tool assembly; adischarge control module that is in data communication with the triggerswitch and regulates power provided to the motor in accordance with aposition of the trigger switch by adjusting a duty cycle of pulse widthmodulated control signal of a switch disposed in series with the motor,wherein the discharge control module is configured to monitor aparameter indicative of tool operation while power is delivered from thebattery pack to the motor and fade the power delivered from the batteryto the motor by adjusting the duty cycle of the control signal by anamount that is computed as a function of a value of the parameter,wherein the discharge control module sets the duty cycle to a valuecomputed as a function of a value of the parameter when the computedvalue of the duty cycle is less than the duty cycle set in accordancewith position of the trigger switch, and retains the duty cycle set inaccordance with the position of the trigger switch when the computedvalue of the duty cycle is more than the duty cycle set in accordancewith the position of the trigger switch.
 9. The power tool system ofclaim 8 wherein the discharge control module fades the power deliveredby decreasing the power delivered as a function of the value of theparameter, where the function is at least one of linear, step,exponential or polynomial.
 10. The power tool system of claim 8, whereinthe discharge control module monitors voltage of the battery, initiatesfading of the power delivered when the battery voltage falls below avoltage fade threshold and decreases the power delivered linearly as afunction of the battery voltage until the battery voltage falls below avoltage cutoff threshold.
 11. The power tool system of claim 8, furthercomprising a temperature sensor disposed in the tool assembly, whereinthe discharge control module is configured to receive a temperaturemeasure from the temperature sensor, initiate fading of the powerdelivered when the temperature exceeds a temperature fade threshold anddecrease the power delivered linearly as a function of the temperatureuntil the temperature exceeds a temperature cutoff threshold.
 12. Thepower tool system of claim 8, further comprising a temperature sensordisposed in the battery pack and in data communication with thedischarge control module, wherein the discharge control module isconfigured to monitor temperature in the battery pack, initiate fadingof the power delivered when the temperature exceeds a temperature fadethreshold and decrease the power delivered linearly as a function of thetemperature until the temperature exceeds a temperature cutoffthreshold.
 13. The power tool of claim 12, wherein the battery packincludes a positive terminal, a negative terminal, a temperature senseterminal, battery cells coupled to the positive and negative terminals,and a temperature sensor electrically coupled to the positive terminal.14. The power tool of claim 8, wherein the switch is disposed in serieswith battery cells of the battery pack.
 15. The power tool system ofclaim 8, wherein the discharge control module resides in the toolassembly.
 16. The power tool system of claim 8, wherein the dischargecontrol module resides in the battery pack.
 17. A power tool systemcomprising: a tool assembly having a motor; a battery pack having aplurality of battery cells that selectively couples to the tool assemblyvia a positive terminal and a negative terminal to provide electricpower to the motor; a trigger switch disposed on the tool assembly; anda discharge control module within the tool assembly that controls powerdelivered from the battery to the motor in accordance with a position ofthe trigger switch, wherein the discharge control module is electricallycoupled to the battery pack to measure voltage of the battery cellswhile the tool is being operated by a user by interrupting current flowfrom the battery pack to the motor momentarily via a power switch whilethe trigger switch is being pulled by the user and measuring voltage ofthe battery in an unloaded condition while current flow is interrupted,the discharge control module being further configured to compare theunloaded voltage measure to a voltage cutoff threshold, and resumecurrent flow from the battery to the motor when the unloaded voltagemeasure exceeds the voltage cutoff threshold.
 18. The power tool ofclaim 17, wherein the discharge control module is configured toterminate current discharge of the battery via the switch when theunloaded voltage measure is less than the voltage cutoff threshold. 19.The power tool of claim 17, wherein the discharge control module isconfigured to interrupt current via the power switch for a period oftime on the order of milliseconds to measure voltage of the battery inan unloaded condition.
 20. The power tool of claim 17, wherein thedischarge control module is configured to delay the voltage measure ofthe battery in an unloaded condition.
 21. The power tool of claim 17,wherein the discharge control module is configured to measure voltage ofa single battery cell or a portion of the battery cells.
 22. The powertool of claim 17, wherein the discharge control module is configured tointerrupt current periodically via the switch to measure voltage of thebattery in an unloaded condition while current is being drawn from thebattery.
 23. The power tool of claim 17, wherein the discharge controlmodule is configured to fade the power delivered from the battery to themotor by an amount that is computed as a function of a value of theunloaded voltage measure.